1. Field of the Invention
This invention relates to a method for focusing during a picture scanning and recording, and more particularly to a method for automatically adjusting focus of images of a recording light beam against changes of distance between a recording drum and an exposure light source.
2. Description of the Prior Art
In recording, the recording drum which is applied with a medium to be recorded such as a film around its outer surface is rotated, and images are produced by a scanning light beam which is projected on the said medium through a projection lens provided on a recording head, and then, they are scanned for recording. In this case, when a large film, for example, 841.times.1189 mm or 1030.times.1456 mm, is required, the recording drum also should have a large diameter in accordance with the size of the medium or the film.
In such a drum as having a large diameter as well as a large width, it is more difficult in manufacturing the drum to keep machining errors of the drum diameter within a range of depth of a focus of the projection lens. Furthermore, changes of the diameter can be caused by changes of conditions in use and/or by changes of its surrounding temperature, which may result in troubles in recording operation.